1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and in particular to a heat-sensitive recording material in which a diazonium salt and a coupler are used as color-developing components.
2. Description of the Related Art
A diazonium salt is a compound that has very high chemical activity, reacts with a phenol derivative or a compound (what is called, a coupler) having an active methylene group, so as to produce an azo dye easily, and further has photosensitivity. Consequently, the salt is decomposed by irradiation with light so that the activity is lost. Thus, diazo compounds have been used for a long time in optical recording materials, a typical example of which is a diazo copying material (see “Basis of Photographic Engineering—Non silver halide photography-”, edited by Society of Photographic Science Technology of Japan and published by Corona Publishing Co., Ltd., (1982), pp.89-117, and pp. 182-201).
Furthermore, recently a diazonium salt has been applied to a recording material in which the fixation of images is required, using the nature that the salt is decomposed by light so that the activity thereof is lost. Typically, proposed is an optical fixation type heat-sensitive recording material, in which a diazonium salt and a coupler are heated and caused to react with each other in accordance with image signals so as to form images, and subsequently the images are fixed by irradiation with light (Koji Sato et al., “Journal of the Institute of Image Electrons Engineers of Japan” Vol. 11, No. 4 (1982) pp.290-296, etc.).
Such a recording material using a diazonium salt as a color-developing component has a drawback that the shelf life of the recording material is short since the chemical activity of the diazonium salt is very high and it thermally decomposes gradually even in a dark place so that the reactivity is lost. The recording material also has a drawback that the diazonium salt compound which remains in the background portion, which is a non-image portion, decomposes at the time of optical fixation so that the generation of the decomposition product (stain), which is colored, causes the non-image portion to be colored. Additionally, the recording material has a drawback that the light resistance of the non-image portion is weak in the recorded material which is completed after the fixation and thus if the recorded material is allowed to stand under sunlight or a fluorescent lamp for a long time, the coloration increases.
As methods for improving such instability of a diazonium salt, various methods have been proposed so far. As one of the most effective methods, there is known a method of encapsulating a diazonium salt into microcapsules. By encapsulating a diazonium salt into microcapsules, the diazonium salt is isolated from decomposing-promoting materials such as water and bases. Consequently, the decomposition thereof is remarkably suppressed. The shelf life of the recording material obtained using this method is also drastically improved (Tomomasa Usami et al., “Journal of the Society of Electrophotography of Japan” Vol. 26, No. 2 (1987), pp. 115-125).
As described above, an ordinary method for encapsulating a diazonium salt into microcapsules is a method of dissolving the diazonium salt into a hydrophobic solvent (an oil phase), adding this to an aqueous solution wherein a water-soluble polymer is dissolved (a water phase), emulsifying and dispersing the resultant mixture in a homogenizer or the like, and further adding a monomer or a prepolymer which will be a wall material of the microcapsules to either of the oil phase or the water phase, or both of the phases. In this way, a polymerization reaction is generated or a polymer is precipitated on the interface between the oil phase and the water phase, so as to form walls of the polymer compound, thereby forming the microcapsules. Such a method is described in details in “Microcapsule” (written by Asashi Kondo, and published by the Nikkan Kogyo Shimbun, Ltd. in 1970), “Microcapsule” (written by Tamotsu Kondo et al., and published by Sankyo Shuppan Co., Ltd. in 1977), and other documents.
Examples of the material used in the capsule walls of the formed microcapsules include crosslinked gelatin, alginic acid salts, celluloses, urea resin, urethane resin, melamine resin, and nylon resin.
In particular, in the case of microcapsules having walls made of a material having a glass transition temperature which is slightly higher than room temperature, such as urea resin or urethane resin, the capsule walls exhibits substance-nonpermeability at room temperature but exhibits substance-permeability at a temperature of glass transition or higher. These microcapsules are called heat responsible microcapsules, and are very useful for heat-sensitive recording materials.
In other words, in a heat-sensitive recording material wherein a heat-sensitive recording layer which comprises, as color-developing main components, heat responsible microcapsules containing a diazonium salt and a coupler outside the microcapsules is disposed on a substrate, the diazonium salt can be stably held for a long time and further color developed images can easily be formed by heating. Additionally, the formed images can be fixed by irradiation with light.
Accordingly, by the encapsulation of a diazonium salt into microcapsules, the stability of the recording material can be drastically improved. However, the instability resulting from the diazonium salt itself cannot be completely suppressed, and a sufficient long-term storability of heat-sensitive recording materials has not yet been obtained.
In recent years, it has been highly desired to reduce the time required for recording an image, that is, accelerate image-formation, which includes the steps of printing an image and fixing the image. In particular, in an optical fixation type heat-sensitive recording material using a diazonium salt, it has been highly desired to develop a technique for improving the above-mentioned stability and acceleration as well. In order to meet the desire, it is an essential requirement to improve the photolysis speed of a diazonium salt itself.
In order to perform optical fixation effectively in such a recording material using a diazonium salt as a color-developing component, it has been general to irradiate an ultraviolet ray having a wavelength of about 360 nm in the fixation step. However, such an ultraviolet ray has problems that an especial light source is required and an effect thereof on eyes is feared. Thus, there has been demanded a recording material using a diazonium salt capable of being effectively fixed by a visible light source having a wavelength longer than 400 nm.
However, with respect to a recording material using a conventional diazonium salt, when inactivated by a light source having a wavelength longer than 400 nm, the recording material has such problems that it is slow and takes a long time for fixation.
Diazonium salts having various counter anions have been known so far. In general, the following are known as inorganic anions: a hexafluorophosphoric acid ion, a fluoroboric acid ion, a chloride ion, a hydrogensulfate ion, and a sulfuric acid ion; and the following are known as organic ions: a polyfluoroalkylcarboxylic acid ion, polyfluoroalkylsulfonic acid ion, an aromatic carboxylic acid ion, an aromatic sulfonic acid ion, a tetraarylborate ion, and so on. In particular, a hexafluorophosphoric acid ion and a fluoroboric acid ion are generally known. However, these have problems of low solubility in organic solvents, low stability, and the like, and, thus, they are not easily used as ingredients of recording materials with advantages. A diazonium salt having a sulfoneimide anion as a counter anion is mentioned in “Journal of Fluorine Chemistry, 2000, 106, 139”, “Inorganic Chemistry, 1993, 32, 223”, “Mendeleev. Commun., 1992, 70”, “Synthesis, 1998, 1171”, “Synthesis, 1999, 90”, and so on. However, these salts have a low solubility in organic solvents and a high water-solubility. Thus, it is difficult to use these salts as ingredients for recording materials with advantages.
Hitherto, diazonium salt compounds with variously altered structures have been proposed in order to solve these problems. However, a diazonium salt having the following three properties has not been discovered: a high storage stability; a high solubility in hydrophobic organic solvents; and a sufficient fixing rate when fixed using a fixing light source having a wavelength longer than 400 nm.